I believe it is not a supported feature since the Mail documentation about this aspect is missing and there's an open issue here.
Anyway, if the mails are handled like XMPP addresses
Each version of an app has its own set
of XMPP addresses in the following
format:
anything@version.latest.app-id.appspotchat.com
If App Engine receives a message for
an address in this format, it is
routed to the corresponding version of
the app (if that version still
exists).
you could try with:
foo@version.latest.app-id.appspotmail.com
EDIT:
This post confirms that you CAN'T do it.